Conventional methods used for disinfection of unpotable water include boiling, filtration, chlorination, ozonation, and ultra violet (UV) treatment. For remote locations, any method of treatment that can utilize solar energy is highly desirable; even more so as it is eco-friendly, sustainable and involves no hazards of any kind.
It is known, that UV part of solar radiation kills and de-activates pathogens in water in the wavelength range 400 nm to 100 nm of UV, and that disinfection of drinking water through de-activation by UV depends on the UV dosage which is proportional to the product of sunshine intensity and water exposure time. One known possible way of intensifying the intensity of solar UV radiation is to use mirrors which reflect to re-direct additional solar radiation to the water treatment chamber. In the case of silver coated glass mirrors, the reflectivity is higher if the iron content of the glass is lower and also if the mirror is thinner. Commercial float glass mirrors with second surface (backside) silver coating are known to have optical reflectivity in the range of 85%-90%. Low iron float glass mirrors are known to have a little higher optical reflectivity in the range of 90%-93%. Polished flat metal mirrors over coated with dielectric layers are available commercially to increase reflection in the entire range of solar spectrum or in a given interval of solar spectrum. For example, aluminum over-coated with quartz is the most commonly used metal for higher mirror reflectivity for the UV, visible and infra red parts of spectrum and aluminum over coated with magnesium fluoride (MgF2) is used for high reflection efficiency of about 96% for the UV range (200 nm to 400 nm) of solar spectrum (for example refer www.hilltech.com/products/uv_components/UV_reflectors.html, www.mellesgriot.com/products/optics/oc—5—1.htm,www.industrial-paints.globalspec.com/FeaturedProducts/Detail/JMLOpticalIndustries/Reflective_Coatings_for_Efficient_Mirrors/27933/0 www.edmundoptics.com/techSupport/DisplayArticle.cfm?articleid=269, www.optarius.com/uv_enhanced_aluminum_mirrors.htm all as on 31/08/08, and Raghunath et al., 2008 Journal of Physics: Conference Series 114)
Reference may be made to the use of solar water heater for disinfection by pasteurization. Commercial solar pasteurizers heat water to a temperature of 79.4 degree C. but such heating leads to relatively less productivity. For example, one commercial solar pasteurizer produces about 30 liters on a sunny day and to avoid scaling in the pasteurizer it needs softening pre-treatment of feed water if it is hard.
Reference may be made to the patent number: NL1023450-C2, Derwent Primary Accession Number: 2005-097533 [11], titled “Stand alone water disinfection unit, contains UV reactor on fluid connection between water storage tank and outlet” by Koehorst Amto, whose drawback is the need for a ultraviolet reactor powered by electricity derived from solar energy using solar photovoltaic cells which are relatively more expensive. In effect, the above cited invention is mainly about substituting grid-supplied electrical power with solar-generated electrical power to provide UV radiation and not the direct use of UV radiation already available in solar spectrum, for disinfection.
Another solar disinfection method is SODIS (Solar Disinfection) which was developed by the Swiss Federal Institute for Environmental Science and Technology (EAWAG). This method uses plastic (PET or PVC) bottles to fill raw water and expose to solar radiation from one hour to two days depending upon the available intensity of sun shine. The main limitations of SODIS is that effective penetration of solar radiation depends on the clarity of the bottle material and of the contained water; solar radiation can sometimes be rather diffused and may not lead to effective knock out of pathogens, and the path length of solar radiation inside water-filled bottle is too long for uniform penetration to all parts. Another yet uninvestigated possibility is re-activation of de-activated pathogens in the warm water under diffused sunshine or overnight. Other risks include infection by the use of unclean bottles and leaching of toxic substances from plastic material of bottles into the water.
Reference may be made to the publication by Acra, A, et al (1980), “Disinfection of Oral Rehydration Solutions by Sunlight”, The Lancet 2:1257-1258, Acra, A., et al., (1990) which mentions experiments on solar disinfection of oral rehydration solution placed in polyethylene bags placed in sunlight for two hours. Even though the effectiveness of UV treatment and also the throughput would be higher at higher intensities, no reference is made either to use of concentrated solar radiation or to the direct exposure of the feed water to the radiation. Further, polyethylene bags degrade with prolonged exposure to solar radiation and due to scratches caused by repeated handling, resulting in less penetration of solar radiation.
Another reference may be made to the publication “Solar water disinfection”, Proceedings of a workshop held at Brace Rice research Institute, Montreal, Canada, 15-17 Aug. 1988, edited by T. A. Lawand et al., which covers extensively different methods of solar water disinfection, their advantages and disadvantages. The main drawbacks in all these processes are either limited productivity of treated water or use of materials such as plastics or glass that reduce transmission of solar ultraviolet radiation to water or deteriorate due to their continued exposure to solar ultraviolet radiation. There is also no reference to the use of concentrated solar radiation.
Yet another reference may be made to the paper by Laurie F. Caslake et al., titled “Disinfection of contaminated water by using solar irradiation”, in Applied and environmental microbiology, February, 2004, p 1145-1150 which describes a method of solar disinfection of drinking water by a continuous flow method by using a grooved polyvinyl chloride plate for continuous flow of water which was covered permanently by an acrylic plate which is transparent to ultraviolet radiation. Besides the fact, that acrylic plates are vulnerable to scratches and can deteriorate due to prolonged exposure to solar ultraviolet radiation, no reference is made either to use of concentrated solar radiation or to the direct exposure of the feed water to the solar radiation.